It is important to secure that an appropriate light illuminance is provided by street and road lighting systems comprising a plurality of outdoor luminaries or luminaires. The light illuminance generated by each luminaire should preferably lie within prescribed limits at the road or ground level to ensure an even and adequate lighting along the road stretch or area in question. However, because of dirt and age depreciation, the illuminance generated by each lamp may vary considerably and differently over time, leading to an uneven light distribution. It is one object of the present invention to provide an illuminance control system that compensates for the depreciation.
LED based outdoor luminaries gain popularity since LEDs provide significant energy savings over conventional light sources. Additionally, a longer life time of LEDs, approx. 5 times the lifetime of existing bulbs, leads to large potential savings in maintenance and service costs of the road lighting system. However, there exist a number of unsolved problems in the application of LED based light sources for outdoor lighting. The long lifetime of LEDs is accompanied with a significant luminance depreciation or decrease of approximately 30% during the lifetime due to aging of the semiconductor material. Moreover, the luminance depreciation over time is unfortunately difficult to predict because it amongst other factors depends on unpredictable environmental conditions such as temperature.
Hence, it would be highly desirable to provide an illumination control system which in a rapid manner is capable of determining or measuring actual light illuminance of each of the outdoor luminaries at a relevant position such as at the road or ground level below the light source. It is also desirable to provide the illumination control system with a mechanism which enables nearly instantaneous correction of the illuminance of each outdoor luminaire after the actual light illuminance at the road or ground level has been measured.
JP 2002/324686 A2 discloses an illuminance monitoring system comprising a plurality of road lamps or luminaries (LA, LB, LC etc.) communicating wirelessly with a travelling motorized service vehicle. The illuminance monitoring system also comprises light sensors detecting the light intensity emitted by each lamp. The light sensor/photo-detector of the lamp monitor circuit is placed inside the lamp monitor attached to the lamp pole. The lamp monitor is measuring and evaluating the light intensity of each of the street lamps to detect normal or abnormal illuminance data for each luminaire and wirelessly transmits that information to the bypassing motorized service vehicle. The bypassing service vehicle records the condition and position of the defective road lamps for later repair.
JP 2005/100765 discloses an illuminance monitoring system comprising a plurality of road lamps or luminaries (A, B, C etc.) communicating wirelessly with a bypassing or travelling patrol car. The illuminance monitoring system includes light sensors of a lamp monitor circuit detecting the light intensity emitted by each lamp. The lamp monitor circuit is attached to the lamp pole.
The position of the light sensor on the lamp pole in the above summarized patent applications raises a number of technical problems since: (i) the light sensor may be exposed to dirt and therefore give a false reading, (ii) as the light illuminance detected at the lamp pole is not a direct measurement of the illuminance at the road level, computing efforts would be required to correlate these illuminances (iii) a sensor placed at the lamp pole is not capable of detecting an unintended, uneven light illuminance at the road (due to uneven dirt depreciation at the lamp house or due to a lamp pole that is accidentally tilted).
The present invention addresses these problems and numerous others as described in further detail below.